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Making It Across the Barrier
Making It Across the Barrier
We spent a lot of time talking about how great of a barrier the phospolipid bilayer is. But even the best barriers aren't perfect - some things are going to be able to get past (remember that it is referred to as semipermeable) and that is actually a very good thing. So let's look at the barrier in closer detail so we can figure out what kind of molecules might be able to slip through.
To reiterate, a phospholipid has two major parts - the hydrophilic head and the hydrophobic tail. When many phosholipids line up in a plasma membrane, there are two layers of phospholipids acting as a barrier (hence bilayer). It is important to remember that these molecules are not bound to one another with anything like a covalent bond. The thing holding phospholipids together is the hydrogen bonding occurring between their polar heads, As we've seen before, hydrogen bonds can be broken, and relatively easily.
So, some things may be able to push past the layer of polar heads on one side. If something is able to make it past the heads (this really isn't too difficult because all of these particles move, remember?), they now have to contend with the nonpolar tails. There are two layers of nonpolar tails in the middle of the cell membrane, and that area is packed with those carbon and hydrogen atoms! There is not a whole lot of space in the tail region, so this actually represents the biggest obstacle to molecules. Getting through the heads is relatively easy. The tails are reinforced.
What Forces Are You Up Against?
What Forces Are You Up Against?
If you're going to try to make it through the army that is those nonpolar tails, it would help if you could disguise yourself as one of them (it's almost like you're dissolving into their ranks - remember, like dissolves like). As a result, nonpolar molecules are more easily able to traverse the nonpolar tail region of a phospholipid bilayer. If you have a charge (like an ion) or even a partial charge (like a polar molecule), good luck getting past these hydrophobic tails!
But polarity isn't the only thing that matters - size of the molecule does too. A small molecule will have a much easier time making it through those hydrophobic tails because it can shove its way into small spaces that exist between the hydrocarbon tails. Big molecules can't maneuver so easily, so they can't generally pass through the membrane, even if they're nonpolar. To recap, only small, nonpolar molecules can make it through the bilayer without help.
When You Just Can't Get Through on Your Own
When You Just Can't Get Through on Your Own
As we saw, large and/or polar molecules have a really tough time trying to enter or leave a cell through its membrane. But there's an issue with that - a lot of crucial molecules are large! Remember all of those macromolecules? Those were often polymers, and often they were quite large. Even the monomers are big compared to what might make it through the semipermeable membrane. So how can we bring in these crucial molecules? Well, they need a little help...
Sometimes (usually) cell membranes have things attached to them. These things might be sugars or receptors for cell signaling, proteins, or cholesterols, for instance. Cell membranes can look quite busy and intimidating, in fact. But what we are most concerned with are proteins. If you need a job done (like things brought into a cell, for instance), it's going to be an enzyme, or protein, that does it.
Let's take a closer look at some of these proteins, specifically 2 different kinds:
Peripheral Proteins
Peripheral Proteins
Peripheral proteins are found on the cell membrane, but are only found near the edges of the membrane (near the polar heads). They do not go into the nonpolar tail region of the cell membrane - that is why they are called peripheral, they stay near the edge. They are important for a lot of things - often they are used for cell signaling - but they are not as dynamic for transport across the cell membrane, so we will focus on the other type.
Integral (aka Transmembrane) Proteins
Integral (aka Transmembrane) Proteins
These are the proteins that we really care about right now. Integral (so called because they are integrated into the membrane) proteins span the entire cell membrane from one side to another. So these proteins do have to live on both sides of the divide - they have to be nonpolar to hang out with the hydrophobic tails and polar to hang out with the hydrophilic heads. This allows them to serve as a path between the outside (extracellular fluid) and the inside (intracellular fluid) of a cell.
For our course, integral proteins are also known as transmembrane proteins because they go across (trans) the membrane. There technically are some differences between the terms, but none we have to clarify for AP Biology. There are multiple kinds of integral proteins, but the ones you should be familiar are listed here.
A. Channel Proteins
A. Channel Proteins
First are channel proteins, these are exactly what they sound like. These are simply channels, or tunnels, linking one side of the barrier to the other. However, these channels are still VERY specific. You need a separate channel for different molecules. So water cannot go through a channel built for potassium ions - you need one for water. An aquaporin is a channel for water and only water - aqua means water and porin refers to a pore, or hole. No energy is required to run this channel - molecules will just move on their own (we will discuss this more).
B. Carrier Proteins
B. Carrier Proteins
Carrier proteins are pretty simple too, honestly. They simply carry solutes from one side to the other of a barrier. This is a lot like a channel protein, but there are some differences to note. Carrier proteins generally have to bond with the molecule they are bringing over the border. Sometimes carrier proteins will trade one kind of molecule for another. Just like the channel proteins, carrier proteins don't require any additional energy in order to move things across the membrane. If a molecule 'wants' to cross (again, more on that shortly), it will help them to do so. Just like channel proteins, carrier proteins are very specific about what they let through. You need a different carrier protein for a different solute.
C. Pump Proteins
C. Pump Proteins
This takes us to our final kind of integral protein we are concerned with: the pump protein. Pump proteins are exactly what they sound like as well - they are pumps. What do pumps do? They use energy in order to move things where they aren't going on their own, or to speed up the process. This is so important to this unit, and I cannot stress that enough. If a cell needs a molecule (like glucose, for example), you don't want to just sit around and hope it comes in. You are going to do whatever you can to make sure that the molecule gets in. Sometimes, you actually have to fight against what the molecule 'wants' to do (again, more on that next).
In the image here, the diamond-shaped solutes don't want to go up, and the circular solutes don't want to come down. However, the pump is forcing them to do exactly that. How can we force them? Well, we have to provide the pump with some energy.
ATP - The Cell's Primary Battery
ATP - The Cell's Primary Battery
In cells, the main source of energy we will see is ATP, or adenosine triphosphate. But I want you to think of it as a simple battery. ATP is a charged battery. When you drain it (using its energy), it becomes ADP, a drained (D doesn't stand for drained, but it might help you remember), battery. ADP stands for adenosine diphosphate.
Don't worry too much about the water molecules and where that yellow phosphate group goes - focus on the battery analogy and ATP as a currency of energy for the cell. The cell will be trading ATP in for goods and services a lot in upcoming chapters, so understanding ATP as a battery is very important.
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